The date and time settings for a post are beneath the post-editing field, just click the "Post Options" link:
This will reveal some more settings for your post, including the date and time, which are on the right-hand side. Once you've selected a new date and time, be sure to publish your post.
With automatic date and time, your posts will now be timestamped with the date and time that they’re published, not the date and time when you first opened the post editor (as has been the case). You can of course still explicitly set the date and time by opening Post Options and switching to Scheduled at instead. Once a post is published, it will keep its date by default, even if you go back even if you go back to edit the post and re-publish.